This invention pertains generally to the manufacture and/or treatment of glycoside products and, in particular, to a method of imparting to such products resistance to discoloration (i.e., darkening) upon exposure (especially prolonged exposure such as from 3 to 6 months or more) to elevated temperatures (e.g., in excess of about 40.degree. C.) and/or upon exposure to alkaline conditions (e.g., at pH's in excess of about 7). Briefly stated, the indicated method entails treating the discoloration-prone (or color-unstable) glycoside product of concern with a relatively small, but effective amount of a borohydride material.
Glycoside materials such as lower alkyl (e.g., methyl, ethyl, propyl, butyl, etc.) glycosides; higher alkyl (e.g., C.sub.6 -C.sub.30 alkyl) glycosides; and the like are known items which are suitable for use in a wide variety of applications ranging from usage as a reactive polyol in the preparation of condensation polymer products such as urethane polymers and polyester resins to utilization as a surface active agent in detergent compositions and other cleansing products, in agricultural treatment compositions, and the like.
Generally speaking, alkyl glycosides are conveniently prepared by reacting an alcohol of the type and chain length which is desired to form the "alkyl" portion of the glycoside of interest with a saccharide reactant (e.g., a monosacchraride such as glucose, xylose, arabinose, galactose, fructose, etc., or a polysaccharide such as starch, hemicellulose, lactose, maltose, melibiose, etc.) or with a glycoside starting material wherein the aglycone portion thereof is different from the alkyl substituent desired for the ultimate alkyl glycoside product of interest. Typically, such reaction is conducted at an elevated temperature and in the presence of an acid catalyst. Various alkyl glycoside products and processes for making same are disclosed in U.S. Pat. No. 2,235,783 (White, issued Mar. 18, 1941); U.S. Pat. No. 2,356,565 (Chwala, issued Aug. 22, 1944); U.S. Pat. No. 2,390,507 (Cantor, issued Dec. 11, 1945); U.S. Pat. No. 2,442,328 (Young, issued Jun. 17, 1947); U.S. Pat. No. 3,219,656 (Boettner, issued Nov. 23, 1965); U.S. Pat. No. 3,375,243 (Nevin et al., issued Mar. 26, 1968); U.S. Pat. No. 3,450,690 (Gibbons et al., issued Jun. 17, 1969); U.S. Pat. No. 3,547,828 (Mansfield et al., issued Dec. 15, 1970) U.S. Pat. No. 3,598,865 (Lew, issued Aug. 10, 1971); U.S. Pat. No. 3,640,998 (Mansfield et al., issued Feb. 8, 1972); U.S. Pat. No. 3,707,535 (Lew, issued Dec. 26, 1972); U.S. Pat. No. 3,721,633 (Ranauto, issued Mar. 20, 1973); U.S. Pat. No. 3,737,426 (Throckmorton et al., issued Jun. 5, 1973); U.S. Pat. No. 3,772,269 (Lew, issued Nov. 13, 1973); U.S. Pat. No. 3,839,318 (Mansfield, issued Oct. 1, 1974); U.S. Pat. No. 3,974,138 (Lew, issued Aug. 10, 1976); U.S. Pat. No. 4,011,389 (Langdon, issued Mar. 8, 1977); and U.S. Pat. No. 4,223,129 (Roth et al., issued Sept. 16, 1980).
In the preparation of alkyl glycoside products, it is not uncommon for such products to develop an undesirably dark coloration during the course of the synthesis and isolation procedures employed. Various procedures have been suggested for improving the color of such dark colored glycoside products including, for example, treatment with bleaching reagents such as hydrogen peroxide; intentional color formation by heat treatment under alkaline conditions followed by removal (e.g., by precipitation, filtration, etc.) of dark colored impurities generated during said treatment procedure; treatment with decolorizing adsorbents such as particulate carbon materials, etc.; and the like. See in this regard, for example, Gibbons' U.S. Pat. No. 3,450,690 which discloses an alkaline heat treatment/separation procedure that can optionally be followed by treatment with bleaching agents such as hydrogen peroxide or by treatment with decolorizing carbons. See also Cantor's U.S. Pat. No. 2,390,507; White's U.S. Pat. No. 2,235,783; Example 1 of Throckmorton et al.'s U.S. Pat. No. 3,737,426; Examples 5 and 10 of Langdon's U.S. Pat. No. 4,011,389; and Example 1 of U.S. Pat. No. 4,472,170 to Hellyer (issued Sept. 18, 1984) for teachings related to the use of carbon adsorbents for the decolorization of various alkyl glycoside products.
Even when glycoside products are originally prepared (or are subsequently decolorized in accordance with one or more of the procedures set forth above) in a fashion which results in initial color characteristics acceptable for certain applications, such products nonetheless commonly exhibit a propensity to discolor (i.e., darken) as a function of time even under relatively mild storage conditions (e.g., at neutral or slightly acidic pH and normal room temperatures, i.e., 20.degree.-25.degree. C.). The propensity to discolor is greatly accentuated (i.e., in terms of the intensity and rapidity thereof) by exposure to elevated temperatures (such as, for example, in the range of 40.degree. to 100.degree. C. or more) and/or exposure to relatively strong alkaline aqueous environments (i.e., pH of 8 to 12). Generally speaking, the extent of discoloration is related to the severity of the pH/temperature/time to which the glycoside product is exposed.
In U.S. Pat. No. 4,557,729 to McDaniel et al. (issued Dec. 10, 1985), the aforementioned problem of color deterioration of glycoside products during storage thereof is discussed and a method for obviating such problem is disclosed which entails first bleaching the glycoside product of interest with an oxidizing agent such as ozone, hydrogen peroxide, hypochlorite salts, etc., and thereafter exposing the resulting bleached glycoside product to a source of sulfur dioxide (e.g., sulfur dioxide gas, sodium sulfite, sodium metabisulfite, sodium hydrosulfite, etc.) to stabilize said glycoside product against color degradation. While the indicated method has been found to be quite effective in stabilizing the color of glycoside products against deterioration or darkening thereof under relatively mild storage conditions (e.g., at pH's in the range of from about 3 to about 7 and at temperatures in the range of from about 20.degree. to about 30.degree. C.), it has also been found to be not nearly as effective (and, in fact, less effective than is desired in many cases) in stabilizing against color deterioration under harsher conditions such as those involving prolonged storage at elevated temperatures (e.g., 35.degree. to 60.degree. C. or more) and those involving relatively high pH (e.g., pH=8 or more) environments, even in situations involving relatively short term/low temperature exposure. Accordingly, it would be highly desirable to provide a method for imparting improved high temperature and/or alkaline color stability to glycoside products which are otherwise prone to darken substantially upon exposure to high temperatures and/or alkaline conditions.